Electronic differential checking heads are known in which two sensing fingers are connected to a pair of pivoted sensing arms, each of which controls a separate analog sensing element. In such a system, the sensing fingers move at a relatively fast rate while the gear rotates. This, in turn, produces two rapidly varying electrical signals at the output of the two sensors which are then coupled to a differential sensing amplifier to provide a difference signal. However, because the electrical signals are changing very rapidly, they cannot be visually read directly for verification purposes; and, in addition, these rapidly changing signals require relatively expensive circuitry to insure accuracy of the sensing system. In the present invention, a mechanical differential checking head is designed so that even though the sensing fingers move over the surface of the gear being tested at a very rapid rate, the difference in movement between the two sensing fingers provides a slowly changing signal that may be visually verified by a voltmeter to give an indication to the operator that the system is operating properly and in addition the slow movement provides a very accurate reading at a relatively low cost.